Oxygenates, such as ethers, have been a part of the U.S. gasoline strategy since the late 1970's. These materials reduce carbon monoxide emissions and unburned hydrocarbons in the exhaust of internal combustion engines. Another advantage of oxygenates is that they have relatively good blending characteristics. Some oxygenates have better blending characteristics than others. For example, the blending vapor pressures of methyl tertiary butyl ether (MTBE), ethyl tertiary butyl ether (ETBE), and tertiary amyl methyl ether (TAME) are lower than methanol and ethanol making them more attractive gasoline components.
MTBE is produced by contacting isobutylene with methanol over an acidic ion exchange resin catalyst. The reaction is exothermic and is conducted in a liquid phase at moderate temperatures. The operating pressure is chosen in order to maintain the reaction in the liquid phase. Major sources of isobutylene feedstock are derived from catalytic cracking and ethylene cracking. Since isobutylene is a by-product of these processes its supply is limited. While in the past MTBE production has depended largely on the availability of isobutylene from refinery operations, isobutylene is available from other sources. For example, isobutylene can be produced by dehydrating tertiary butyl alcohol (TBA). Isobutylene can also be produced by: (1) dehydrogenating isobutane; (2) isomerizing mixed butenes to isobutylene; or (3) isomerizing mixed butenes to isobutane and then dehydrogenating to isobutylene. These methods of producing isobutylene have proven to be very expensive.
Side reactions that occur in the MTBE reaction zone include (1) the reaction of isobutene with water to form tertiary butyl alcohol (TBA); (2) reaction of one methanol molecule with another methanol molecule to form dimethyl ether (DME) and water; and (3) the reaction of one isobutylene molecule with another isobutylene molecule to form di-isobutylene (DIB).
TAME is produced by reacting methanol with tertiary olefins having five carbon atoms (isoamylenes). As a result, there is an improvement of the octane value of the C.sub.5 olefinic fractions. The octane is increased by four octane numbers, the olefinic content is reduced by a factor of 2, and motor fuel production from the C.sub.5 fraction is increased by 7%. The main sources of isoamylenes are steam-cracked and light FCC gasolines. The isoamylene content of a stabilized steam-cracked C.sub.5 cut and an FCC C.sub.5 cut are 25 wt. % and 30 wt. %, respectively.
Side reactions that can occur in the TAME reaction zone include: (1) the reaction of one isoamylene molecule with another isoamylene molecule to form diisoamylene (DIA); (2) the reaction of one methanol molecule with another methanol molecule to form di-methyl ether (DME) and water; and (3) the reaction of isoamylene with water to form tertiary amyl alcohol (TAA).
Although the reactions and side reactions of MTBE and TAME may appear similar, the equilibrium and kinetics for the formation of MTBE and TAME are strikingly different. The reaction rate constants for TAME are found to be generally less than 50% of the reaction rates for MTBE. Even more significant is the change in equilibrium. Because of this equilibrium difference, the preparation of MTBE from stoichiometric proportions of isobutene and methanol can achieve conversions between 85% and 100%, whereas comparable preparations of TAME have been found to achieve conversions of only 50% to 60%.
The equilibrium conversion for alkyl methyl ethers was significantly improved by the application of reactive distillation technology to the etherification process, i.e., simultaneous etherification and separation by distillation of the etherification product. Reactive distillation is especially suited to the production of alkyl methyl ethers by the reaction of alcohols and isoolefins because the conversion rates for these reactions are equilibrium limited, particularly the TAME reaction. By removing the alkyl methyl ether product from the reaction zone as soon as it is produced, more alkyl methyl ether product is produced, thereby increasing the conversion of isoolefins to ethers.
The application of reactive distillation to etherification was first disclosed in the Haunschild patents (U.S. Pat. Nos. 3,629,478, 3,634,534, and 3,634,535). It was later commercialized using a specific type of catalyst structure disclosed in U.S. Pat. Nos. 4,336,407 and 4,250,052.
U.S. Pat. No. 4,413,150 (Briggs) discloses an etherification process wherein isobutylene and methanol are fed into an etherification zone. The effluent from the etherification zone is sent to a fractionation column which splits the effluent stream into an overhead stream comprising isobutylene, methanol and some MTBE and a bottoms stream comprising MTBE. The overhead stream is taken off the fractionation column at a location above the point where the feed to the fractionation column is introduced and is recycled to the etherification reactor.
U.S. Pat. No. 4,182,913 (Takezono et al.) discloses a method for continuously producing MTBE in which isobutylene and methanol are reacted in the presence of an acidic cation exchange resin in an etherification reactor. The resulting effluent is neutralized and then passed to a separation column. At a location above the point where the feed to the separation column is introduced, an overhead stream comprising isobutylene is recycled to the etherification reactor.
U.S. Pat. No. 4,310,710 (Torck et al.) discloses a process for producing MTBE. In the first step, methanol and isobutylene are introduced into an etherification reactor to form an etherification effluent stream containing MTBE, isobutylene, and methanol. This effluent stream is passed to a fractionation column where a process stream containing isobutylene and MTBE is taken off at a location above the point where the feed to the fractionation column is introduced and recycled to the etherification reactor.
Nevertheless, there is still a need for higher conversion etherification processes, particularly in the case of TAME production.